The present invention relates generally to clock recovery circuits and methods and in particular to systems and methods performing clock recovery for return-to-zero (RZ) serial data stream.
Modern communication systems transfer tremendous amounts of data at ever increasing rates. Various transmission schemes and encoding of data are utilized to assist in the transfer. One particular type of encoding scheme for digital data transmission is a return-to-zero (RZ) data stream. A RZ data stream transmits digital data in which a logical one and a logical zero are determined based on voltage pulses having certain or specific characteristics. In particular, a logical one or zero is determined by the voltage during a first half of each bit. The signal returns to a resting state, typically zero volts, during the subsequent or remaining half of the bit. Additionally, there are two types of RZ data, a positive logic RZ data and negative logic RZ data.
In positive-logic RZ data, a logical zero is represented by a voltage that is more negative or less positive then the voltage that represents a logical one. Conversely, in negative-logic RZ data, a logical zero is represented by a voltage that is more positive or less negative then the voltage that represents a logical one.
Another type of encoding scheme for digital data transmission is a non-return-to-zero (NRZ) data in which a logical one and zero are determined by specific and constant direct-current (DC) voltages. Similar to RZ data, NRZ data has two forms, a positive-logic NRZ data and a negative-logic NRZ data. In positive-logic NRZ data, a logical zero is represented by a voltage that is more negative or less positive then the voltage that represents a logical one. Likewise, in negative-logic NRZ data, the logical zero is represented by a voltage that is more positive or less negative then the voltage that represents a logical one.
Typically, the data stream transmitted in either form, as RZ or NRZ data, is transferred optically and then converted to electrical signals. Subsequently, clock and data information are retrieved from the converted electrical signals. However, optimizing the retrieval of clock and data information from the converted electrical signals is sometimes difficult.